The Hall-Heroult process was first used commercially around 1890. In this process, aluminum is extracted by electrolyzing aluminum oxide, Al.sub.2 O.sub.3, (also known as "alumina") dissolved in a molten salt bath based on cryolite, Na.sub.3 AlF.sub.6 and other additives. The molten cryolite is operated at a high temperature, generally within the range of 940.degree.-1000.degree. C. The alumina is dissolved in the bath and electrolyzed using carbon anodes according to the reaction: EQU Al.sub.2 O.sub.3 +3/2C.fwdarw.2Al+3/2CO.sub.2
In the electrolytic cell, a carbon lining within a crucible typically serves as the cathode; and the anodes, typically carbon, are immersed in the molten salt. The cryolite-aluminum oxide serves as the electrolyte solution. A large electric current in the cell supplies thermal energy that melts the cryolite, which dissolves the aluminum oxide. Aluminum is electrolyzed in the molten state, in which state it collects in the bottom of the cell, also serving as the cathode.
The Hall process is beset with numerous disadvantages, however, which have not been completely solved despite over a century of commercial use. Operating the salt bath used in the process at high temperatures, typically around 970.degree. C., requires large amounts of energy. Attempts at operating the salt bath at lower temperatures by progressively lowering bath weight ratios below the 1.1:1 NaF to AlF.sub.3 bath ratios typically used have been frustrated by the formation of a crust of frozen electrolyte over the molten aluminum as electrolysis proceeds. This crust causes deposition of sodium. which in turn hampers current efficiency and drastically increases resistance at the cathode and reduces metal coalescence to the point that the cell can no longer be operated.
Another serious drawback to the conventional Hall cell technology is the lack of an adequate construction material to contain the molten bath. At the high cell temperatures necessary to maintain alumina in solution, the electrolyte and molten aluminum progressively react with most ceramic materials, creating problems of containment and cell design. As a result, the smelting cells are operated with a high heat loss to produce a frozen layer of bath all around the sides and top of the cell, which protect the cell's graphite-lined sidewalls from corrosion. This mode of operation causes a high energy expenditure and imposes difficulties in operating near the freezing point of the bath because of large variabilities in the thickness of the frozen ledge for small differences in bath temperature.
Other disadvantages of the Hall cell include sodium intercalation and formation of sodium aluminum oxide which causes heaving and cracking of the cell lining with resulting interference in operating characteristics of the cell and shortened cell life.
Numerous methods have been attempted to overcome some or all of the above shortcomings of the Hall process. While many of these methods have met with some success, none has replaced the conventional Hall process in commercial applications. One attempt has been to utilize so-called "low temperature" salt baths which allow reduced energy consumption at the expense of lower alumina solubility. For example, U.S. Pat. No. 3,951,763 discloses a low temperature salt bath and uses a carbon anode, which is consumed in the process. U.S. Pat. No. 3,996,117 adds 5-10 wt. % LiF to the bath.
One of the drawbacks of the low temperature salt bath technology to date has been the realization that reduction of salt bath temperature likewise leads to reduction of alumina solubility. Attempts to overcome this problem include those disclosed in U.S. Pat. No. 3,852,173 wherein the alumina is provided with a sufficient water content to prevent anode dusting, which water content also assists in dispersing the alumina into the low temperature salt bath solution of NaF/AlF.sub.3. However, providing the water-containing alumina is an added requirement of the process and naturally incurs added expense.
Various attempts have been made to utilize so-called "inert" anodes in order to improve the Hall process. See, e.g., U.S. Pat. Nos. 3,718,550; 3,960,678; 4,098,669; 4,233,148; 4,454,015; 4,478,693; 4,620,905; 4,620,915 and 4,500,406. Attempts have also been made to use inert anodes with low temperature salt baths. U.S. Pat. No. 4,455,211 discloses a low temperature salt bath of NaF/AlF.sub.3 which teaches the addition of 1-15% LiF and an inert anode made of an interwoven matrix. LaCamera et al U.S. Pat. No. 5,015,343 discloses the use of an inert anode in connection with a metal chloride and/or metal fluoride salt bath using additives for low temperature aluminum electrolysis. However, this reference teaches alumina solubilities less than 0.6% and the need to increase the anode surface area by 2 to 15 times the superficial, or projected, anode area. Such increased surface area anodes are typically fabricated, for example, by drilling numerous holes deep into the anode.
U.S. Pat. No. 4,681,671 discloses a low temperature salt bath which is used in conjunction with an anode having a relatively large surface area and low current density. Indeed, this reference teaches the necessity of utilizing a low current density and increased anode surface area in conjunction with low temperature salt baths.
A great advantage would be gained if a bath composition could be found which was not corrosive to conventional lining materials and contained favorable attributes for electrolysis; namely, high conductivity, appreciable alumina solubility and low melting point. This would permit elimination of the frozen ledge and allow for more thermally efficient cell designs which are more economical than those presently used. Greater operating flexibility would also be gained because cells could be operated comfortably above the electrolyte melting point, making cell temperature less critical. Until this time, no suitable bath substitutes could be found to fulfill these requirements.